TDM Audio 32CL-2 v.2 - Manual (Page 19)

© 1996 TDM Design, Inc. Compressor/Limiter Owner’s Manual Page 19
An example of a common use for this kind of compression is low frequency emphasis in dance
clubs. A compressor is often used to make sure that the sound level does not damage the
hearing of club patrons or burn out the high frequency drivers in the sound system, but low
frequencies are generally not as damaging to ears as high frequencies, and many dance clubs
have a lot of low frequency power in the sound system, so they don’t want the thundering low
frequency content of dance mixes to drive the sound system into compression constantly. For
such an application, a side chain is the natural choice. Low frequencies are reduced in the side
chain equalizer, and the result is that the compressor is less likely to reduce the output gain
due to an abundance of low frequency content in the signal. If you use a compressor with a
side chain for this kind of protection, however, be aware that you are not providing any pro-
tection in the band of frequencies reduced on the side chain equalizer. You would be wise to
set the limiter threshold to something lower than the absolute maximum that your sound sys-
tem (or your listeners) can take.
•
Automatic Dimming of Program Material: Suppose you have a paging system that plays
music when nobody is speaking on the microphone. Instead of background music, however,
this is foreground music, and it is fairly loud. When somebody needs to speak on the micro-
phone, you don’t want the music to just stop while the speaking is taking place because that
would sound too unnatural. Instead, you would like it to just get quieter so that the person
speaking can be heard, and then to go back up in volume automatically when the speaker fin-
ishes.
This is a fairly common scenario, and the solution is side-chain compression. This kind of side
chain compression is unusual, however, because the program material itself is not actually
processed in the side chain. The side chain send is simply ignored, and a signal from the mi-
crophone preamplifier or mixer is fed into the side chain return. With this setup, the level of
the microphone signal determines the amount of gain reduction. The louder the signal at the
microphone, the quieter the program material gets. The threshold is set fairly high so that am-
bient noise on the microphone does not reduce the level of the program material. The
compression ratio, however, is typically quite high (20:1 or more) so the volume reduction is
dramatic enough to permit the speaker to be heard. The attack time is set to fast so that the
volume is reduced immediately when someone starts talking on the microphone. The release
time, however, is set to slow so that the volume does not go back up when the person pauses
to take a breath. When they are finished speaking, the volume of the program material gradu-
ally ramps back up to normal.
This kind of side chain compression is common in clubs with DJs, and in radio and television
broadcasting.
Connections for Side Chain Operation
Each channel has a side chain connector. This connector is a tip-ring-sleeve type ¼” phone plug.
The side chain send and return share a common ground which is connected to the sleeve. The tip
is the signal send for the side chain and the ring is the signal return.
To use a side chain, make an insert cable (see Using Channel or Subgroup Inserts), and plug its
tip-ring-sleeve connector into the side chain connector on the back of the TDM 32CL series
compressor/limiter. Plug the connector on the other end that corresponds with the tip into the